The output stage of class-AB amplifiers operates by using a current source and a current sink. This configuration is sometimes referred to as a “push-pull” configuration since a first branch of the output stage “pushes” or sources currents to a load while a second branch of the output stage “pulls” or sinks current from the load. Class-AB amplifiers avoid the high power consumption of a class-A amplifier by always having one output branch substantially turn off when the other output branch is turned on. Although the current in one leg of a class AB amplifier is substantially turned off there is a small amount of current flowing in that leg. The small residual current in the class AB amplifier avoids the crossover distortion produced the turning on and off of the currents in class-B amplifiers. Thus class-AB amplifiers are able to achieve a relatively high current output while maintaining a low quiescent current. The currents in class-AB amplifiers are inversely related such that when one current becomes large, the other current becomes very small. When a current becomes small, it may disadvantageously result in a “cut-off” in one of the output transistors, which causes the undesirable crossover distortion.
This problem has been addressed in amplifiers by using the harmonic mean principle that is described by the equation z=x*y/(x+y), where x and y represent push and pull currents, respectively, and z represents a bias current. From this relationship it can be seen that as a push current becomes larger (represented by x here, for example), the accompanying pull current will become smaller, but will not result in a “cut-off” of the transistor carrying the pull current because the pull current can never become smaller than the bias current. Likewise, a large pull current will be accompanied by a small push current, which also can never become smaller than the bias current.
Bipolar devices have been used to implement the harmonic mean principle in class-AB output stages. However, the relatively limited beta available from bipolar transistors disadvantageously restricts the suitability of a bipolar implementation for applications requiring a large push/pull current to quiescent current ratio. Proposed topologies, for MOS devices such as those discussed in Hogervost, R. et al., “A compact Power-Efficient 3V CMOS Rail-to-Rail Input/Output Operational Amplifier for VLSI Cell Libraries,” IEEE JSSC 29(12):1718 (December 1994), are unsuitable for applications requiring a low supply voltage because they require a supply voltage of at least 2*VGS+VDSsat.
Therefore it can be seen that there remains a need for the high gain class-AB output stage that is able to operate using a low-voltage power supply. The present invention provides this and other advantages as will be apparent from the following detailed description and accompanying figures.